Power-over-Ethernet (PoE) is described in the most recent standard, IEEE Standard 802.3af-2003, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, Amendment: Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI); IEEE Computer Society. By means of PoE, power is supplied to detached data equipment and peripherals (like routers, switches, printer spoolers etc.) through the same wire or network connection that is already used to connect these to the Ethernet (network). There are plans to make use of the same standard for all kinds of low power loads such as, e.g., lighting equipment (sensors, switches, light sources etc.) or entertainment appliances like active speakers, internet radios, DVD players, set-top boxes and even TV sets. Standardization in IEEE Standard 802.3 is proceeding to support power levels of, e.g., up to 100 W per Cat5/6 connection.
As PoE supply systems for power distribution inside buildings come into the focus of the industry, some specific usage aspects of these networks need to be solved in order to get these direct current powered networks (so called “DC-Grids”) widely employed. Historically, when the PoE standard was introduced for supplying power to detached networking devices like routers, switches, printer spoolers, etc., it was a replacement for small power-plug type power supplies, since the originally intended loads where mostly already equipped with communication and processing means.
As indicated above, an example of powered devices may be light sources, but also sensors or actuators. Typically, two links between power sourcing equipment (PSE) and powered device (PD) are foreseen. A power delivery link is established after detection and classification. Subsequently, a data link is established for the traditional Ethernet data.
In standardization activities for PoE one aspect that is of interest relates to power allocation. The 802.3af standard allowed a PSE to distinguish between several power classes and the 802.3 at standard added support for an additional power class as well as continuous negotiation between PSE and PD. The ability for the PSE to query the PD in order to determine the power requirements of that PD is called classification. The interrogation and power classification function is intended to establish mutual identification and is intended for use with advanced features such as power management. There are two forms of classification: Physical Layer classification and Data Link Layer classification. Physical Layer classification occurs before a PSE supplies power to a PD when the PSE asserts a voltage onto the physical interface and the PD responds with a current representing a limited number of power classifications. With Data Link Layer classification, the PSE and PD communicate using the Data Link Layer Protocol after the data link is established. The Data Link Layer classification has finer power resolution and the ability for the PSE and PD to participate in dynamic power allocation wherein allocated power to the PD may change one or more times during PD operation.
The improved classification features in the 802.3 at standard can be used, compared to the 802.3af standard, to prevent a PSE from allocating too much power to a PD, i.e. a power limit that the PD will never use, having as a consequence that the overage is reserved without due reason. However, a more advanced power allocation approach is required to further optimize the power allocation.
A new standard is being developed for PoE, the 802.3bt standard, which will introduce a new classification mechanism: autoclass. Autoclass is a classification mechanism that allows a PD to communicate its effective maximum power consumption to the PSE. This happens in such a way that the PSE will be able to set the power budget to the effective maximum PD power including the effective channel losses (e.g. losses realized over the Ethernet connection between PSE and PD). This will allow more efficient use of the Power Supply Unit (PSU) of the PSE since only the effectively used power needs to be budgeted. Autoclass is performed as part of the classification process, where a PSE determines what class to assign to a connected PD. In autoclass mode the PD consumes maximum power for a short period during classification and the PSE measures the power consumed. The PSE will then allocate power to the PD based on this measurement.
The presentation “Autoclass II v160” in the context of the “IEEE P802.3bt 4-Pair Power over Ethernet (4PPoE)—Jan. 2015 Interim Meeting” from Jan. 14-16, 2015 in Atlanta, Ga., USA et al discloses an introduction to parts of the above mentioned autoclass principles.
The presentation “IEEE P802.3bt Mutual Identification” in the context of the “IEEE P802.3bt 4-Pair Power over Ethernet (4PPoE)—Sep. 2014 Interim Meeting” from Sep. 10-12, 2014 in Kanata, OT, Canada discloses a mutual identification scheme for both the PD and PSE to learn the capabilities/requirements of the other.